Dip molded products such as gloves and fingerstalls used in the fields of medicine, hygiene and in production site of electronic parts should be superior in oil resistance, mechanical strength, free of pinholes and excellent in close fitting and soft feeling thereof to the skin.
As the method of dip molding, there are known an anode coagulant dip process wherein a mold made of wood, glass, ceramics, metal or plastics is dipped in a coagulant solution and then dipped in a latex compound for a dip molding which comprises natural rubber latex or synthetic rubber latex, vulcanizing agent, vulcanization accelerator, vulcanization auxiliary, pH adjusting agent and the like, and a Teague dip molding process wherein a mold is dipped in the latex compound for dip molding and then dipped in a coagulant solution.
The thickness of a dip molded product, the occurrence of pinholes therein and the good close fitting and soft feeling to the skin depend mainly on the components of latex and a coagulant solution.
Most of latex used heretofore in the dip molding method is natural rubber latex or synthetic rubber latex, but for the reason of high resistance to oil and organic solvent, acrylonitrile butadiene rubber (NBR) latex is preferably used. Recently, owing to the development of allergy in those who use natural rubber latex, the NBR latex has been used more and more. However, molded products produced from NBR by dip molding, though being excellent in oil resistance, have high glass transition temperature (Tg), and thus they hardly give soft feeling to the skin.
Accordingly, there are methods wherein, for instance, a conjugated diene compound such as butadiene is used in a larger amount in production of NBR, or NBR is blended with natural rubber, or laminated on the natural rubber layer. In these cases, however, the resulting products are poor in oil or chemical resistance
Meanwhile, as the method of obtaining NBR film which is excellent in oil resistance and has soft feeling to the skin, the methods wherein e.g. the molecular weight of latex and the content of insolubles in methyl ethyl ketone therein are proposed (JP-A 5-247266 and JP-A 6-182788), but the purpose has not been fully achieved.
For the same purpose, a method of using high-molecular latex having low gel content (JP-A 5-86110) has also been proposed, but the resulting molded product is yellowed and not suitable for medical purposes where white molded products are required.
In addition to the soft feeling to the skin, the absence of pinholes is important for the molded product. In particular, when the molded product is used for medical purposes, the presence of pinholes is fatal. The pinhole is easily generated if bubbles are mixed in latex, the chemical stability (salt coagulability) of the latex to a coagulant solution is inappropriate, or the wetting properties of the latex on a mold are not satisfactory. If such salt coagulability is inappropriate or the wetting properties of the latex on the mold are not satisfactory, then the thickness of the molded product may become uneven and pinholes may be generated.
It is also important for the molded products that the surface of the product is not sticky. This stickiness is a property permitting molded products to adhere one another owing to the sticking property on the surfaces of the molded products. Once the molded products assume stickiness on their surface, their qualities are lowered, resulting in a significant reduction in their commercial value.
Further, if the chemical stability (salt coagulability) of the latex to the coagulant is inappropriate, dip molded products produced by a Teague coagulant dip process wherein a mold is dipped in a latex and then dipped in a coagulant solution, undergo shrinkage of the molded product resulting in a significant reduction in their commercial value.